Yellow light is viewed by reflection from a thin vertical soap film. Let $\lambda$ be the wavelength of the light within the film. Why is there a large dark space at the top of the film?
A) no light is transmitted through this part of the film
B) the film thickness there is$\lambda$/4
C) the light reflected from exactly one of the two surfaces undergoes a 180 $\degree$ phase change
D) the film is too thick in this region for thin film formulas to apply
E) the reflected light is in the infrared

Question

Yellow light is viewed by reflection from a thin vertical soap film. Let $\lambda$ be the wavelength of the light within the film. Why is there a large dark space at the top of the film? 
A) no light is transmitted through this part of the film 
B) the film thickness there is$\lambda$/4 
C) the light reflected from exactly one of the two surfaces undergoes a 180 $\degree$ phase change 
D) the film is too thick in this region for thin film formulas to apply 
E) the reflected light is in the infrared

Quizplus · Accepted Answer

The large dark space at the top of the film is caused by destructive interference resulting from the reflection of the yellow light from the top surface of the film and the bottom surface of the air above it. Because the path difference between the two waves is equal to the thickness of the soap film, the light reflected from exactly one of the two surfaces undergoes a 180-degree phase change, leading to destructive interference and the observed dark space.

Yellow Light Is Viewed by Reflection from a Thin Vertical λ\lambdaλ

Yellow Light Is Viewed by Reflection from a Thin Vertical $\lambda$